The study concerns a type-C virus which has been isolated from two human diploid fibroblast cell strains. It is planned to continue experiments on the in vitro expression of this HEL-12 virus in human cells as a function of time in culture, to continue work on the antigenic relationship of this virus to other type-C viruses, and to compare the spontaneoulsy released type-C virus with a type C virus induced by iododeoxyuridine.